In practice, it may sometimes be desirable to increase use of plant-derived processing aids for preparation of rubber compositions by reducing use of petroleum based processing aids for such purpose.
This invention provides a way to utilize a plant-derived rubber processing aid in a rubber composition which contains silica reinforcement.
In practice, petroleum based rubber processing oils are often used to aid in the processing of unvulcanized rubber compositions by reducing their mixing viscosity (e.g. Mooney viscosity).
While non petroleum, plant-derived rosin acids may be candidates for unvulcanized rubber processing aids, the high acid group content of rosin acids have an effect of retarding amine accelerated sulfur cure of the associated rubber composition (because of interaction of its high level of acid groups with amine based sulfur cure accelerators) which significantly limits the suitability of use of rosin acids for such purpose.
Decarboxylated rosin acids (with significantly reduced acid group content) have been suggested for use as processing aids for carbon black reinforced rubber compositions (which can have a reduced effect of retarding the sulfur curing of the rubber composition because of their reduced acid group content). For example, see U.S. Pat. No. 4,478,993.
However, use of partially decarboxylated rosin acids (which thereby retain a portion of their acid groups because they are not completely decarboxylated) in rubber compositions which contain silica reinforcement are envisioned herein to promote significant advantageous effects, particularly for components for articles of manufacture such as tires.
For this invention, liquid partially decarboxylated rosin acids, which might be referred to as “rosin oils”, are provided as processing aids (to reduce unvulcanized rubber mixing viscosity) in rubber compositions which contain silica reinforcement where the partially decarboxylated rosin acids contain a range of acidity values (range of acid group content) for which its acid groups are relied upon to interact with hydroxyl groups (e.g. silanol groups) contained on synthetic, amorphous silica (e.g. precipitated silica) and to thereby provide a bridging network between the silica and the rosin oil within a vulcanized rubber composition with the rosin oil thereby being in a sense anchored to the silica.
In this manner, then, it is envisioned that a tertiary network is formed in situ within the rubber in addition to a silica/rubber network and silica/silica coupler network not found with a rubber composition which is reinforced solely with rubber reinforcing carbon black filler.
Accordingly, such rubber composition is envisioned as containing a bridge network comprised of a product of an interaction of acid groups of said partially decarboxylated rosin acid with hydroxyl groups of said precipitated silica.
Further, rosin acids themselves are normally in a nature of being solid materials in a sense of having a softening point of, for example, about 85° C. and therefore solid at room temperature (about 23° C.) which are considered herein as not significantly improving the processing effect of petroleum based rubber processing oils for already mixed unvulcanized rubber compositions such as, for example, for shaping (e.g. extruding) prepared rubber compositions.
As previously discussed, the significantly high acid group content of the rosin acids (e.g. acid number of about 160) has been observed to retard the rate and state of sulfur cure of conjugated diene-based rubber compositions, where an amine based sulfur cure accelerator is used, which is typically an unwanted feature. The acid number of rosin acids, as well as partially decarboxylated rosin acids, may be calculated based upon milligrams of KOH (potassium hydroxide contained in a water solution) required to neutralize 1 gram of the rosin acid. For example, it typically takes about 160 milligrams of KOH to neutralize one gram of a typical rosin acid to therefore yield an acid number of 160.
In contrast, for this invention, it is believed that use of non-petroleum, plant derived partially decarboxylated rosin acids (for example, partially decarboxylated wood rosin acids) in a sense of being liquid and having a significantly reduced acid number as a total or partial replacement for conventionally used petroleum based rubber processing oils in a rubber composition which contains silica reinforcement is novel and a significant departure from past practice.
Historically, rosin acid is a solid resinous material which contains a high concentration of carboxylic acid groups which occurs naturally in pine trees. There are three major exemplary sources of the rosin acid, namely:
(A) gum rosin acid, which might be referred to as “rosin”, is from the oleoresin extrudate of the living pine tree,
(B) wood rosin acid, which might be referred to as “wood rosin” or “rosin”, is from the oleoresin contained in the aged pine tree stumps; and
(C) tall oil rosin acid, which might be referred to as “rosin”, is from the waste liquor recovered as a by-product in the paper production industry.
For example, wood rosin may be obtained from aged pine tree stumps. In such practice, a pine tree stump may be allowed to remain in the ground for a number of years so that its bark and sapwood may decay and slough off to leave the heartwood rich in rosin acid, which might sometimes be referred to as “wood rosin”.
Historically, rosin acids derived from both oleoresin and aged pine tree stump wood are typically composed of, for example, about 90 percent rosin acids and, for example, about 10 percent nonacidic components.
Representative of various wood rosin acids are, for example, rosin acids referred to as abietic, levopimaric, neoabietic, palustric, dehydroabietic, dihydroabietic, tetrahydroabietic, pimaric, isopimaric, elliotinoic and sandaracopimaric.
Liquid decarboxylated rosin acids (rosin oils) have been produced by the decomposition of rosin acids at high temperatures. For example, the rosin oil (decarboxylated rosin acid) may be produced, for example, by the treatment of rosin acid at an elevated temperature which may optionally be, for example, in the presence of hydriodic acid or iron turnings.
According to the concepts of the present invention, it has been discovered that a type, or class, of non petroleum, naturally occurring plant-derived rosin acids can be partially decarboxylated to form a rosin oil and then be partially or fully substituted for petroleum based rubber processing oils utilized in various rubber compositions. Such various petroleum based rubber processing oils include, for example, aromatic oils, naphthenic oil, paraffinic oils, and blends thereof.
Rubber processing oils may be composed of, for example, naphthenic and alkylated naphthenic hydrocarbons and mixtures thereof with various aromatic hydrocarbon containing oils which are well known to those having skill in such art. They may comprise distillate fractions boiling above about 200° C. Suitable fractions are those at least 90 percent of which boil above about 250° as more volatile members may be lost during or after compounding and curing the rubber.
The present invention further contemplates the use of partially decarboxylated plant-derived rosin acids, particularly which have an acid number of no more than 30 as a partial or full replacement of customary petroleum based rubber processing oils (including petroleum based oils rubber extending oils) in rubber compositions.
The carboxylic acid containing rosin acids themselves are, in general, a composite of a mixture of molecules, some of which contain monocarboxylic acids typically comprised of a general molecular formula such as, for example, C20H30O2. The names of various individual rosin acids has varied somewhat over the years. For example, in addition to trivial names, names such as for example, abietic acid have sometimes been used.
The carboxylic acid containing rosin acid molecule contained in the rosin acid composite of rosin acids, in general contains two chemically reactive moieties, namely a carboxylic acid group and carbon-to-carbon double bonds which might sometimes be referred to more simply as “double bonds”. Through such reactive moieties, many modifications in structure and numerous derivatives are obtainable for the rosin acids. Because an individual rosin acid is typically a composite composed of a number of rosin acid molecules and a number of molecules which do not contain carboxylic acids, the chemistry of its reactions is relatively complex.
Therefore, in addition to various carboxylic acid related chemical reactions, double bond related reactions or interactions may also occur.
The rosin acid's carboxyl group is typically of a structurally hindered nature which typically makes it necessary to use high temperatures or other significant conditions to bring about its decarboxylation.
The present invention relates to the use of liquid partially decarboxylated rosin acid as a partial or complete replacement of petroleum based rubber processing oils, including rubber extender oils, in rubber compositions, particularly for tire components, which contain silica reinforcement. Its usefulness relies, in part, upon an interaction between acid groups of the partially decarboxylated rosin acid and hydroxyl groups of the silica reinforcement to form a bridging silica network within the rubber composition.
In such practice of using liquid partially decarboxylated rosin acid in silica reinforcement-containing rubber compositions, particularly for tire components and particularly for tire treads, considerations such as for example acid number (carboxylic acid content) are a factor to be taken into consideration. As a result, for this invention, a liquid partially decarboxylated rosin acid (rosin oil) of a low acid number, (significantly lower than a typical acid number for rosin acid such as, for example, about 160) in a range of from about 2 to about 30, alternately from about 4 to about 15, is provided for promoting a combination of a beneficial interaction effect with the silica reinforcement within the rubber composition to promote cured rubber properties as well as a beneficial rubber processing effect for the uncured rubber composition without significantly affecting the rubber composition's rate and state of cure.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, where used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.
The term “rosin oil” may be used to refer to a partially decarboxylated rosin acid.